1. Field of the Invention
The present invention relates to an electrophotographic type or electrostatic recording type image forming apparatus, such as a copying machine or a laser beam printer, and to a process cartridge to be used in such an image forming apparatus.
2. Description of the Related Art
In electrophotographic type and electrostatic type image forming apparatuses, a corona charger has conventionally been used as the charging means for the image bearing member consisting of an electrophotographic photosensitive member, electrostatic recording dielectric member or the like.
Recently, due to its advantages, such as low ozone and low power consumption characteristics, a contact charging device has been put into practical use, in which a charging member to which voltage is applied is brought into contact with the object to be charged, such as an image bearing member, to thereby charge the object to be charged. In particular, from the viewpoint of stabilization in charging, a roller charging type device using a conductive roller as the charging member is preferred.
In the roller charging type contact charging device, a conductive elastic roller serving as the charging member is brought into press contact with the object to be charged, and voltage is applied thereto to thereby charge the object to be charged.
More specifically, charging is effected through discharge from the charging member to the object to be charged, so that, by applying a voltage of not lower than a certain threshold value, charging is started.
For example, when an electrophotographic OPC photosensitive member having a thickness of 25 xcexcm is used as the object to be charged and a charging roller is brought into press contact therewith to effect charging, application of a voltage of approximately 600 V to the charging roller causes the surface potential of the photosensitive member to start to rise. Thereafter, the surface potential of the photosensitive member increases and is linearly inclined with respect to the applied voltage.
In the following, this threshold voltage will be referred to as the charging start voltage Vth. To obtain the requisite photosensitive member surface potential VD for electrophotography, it is necessary to apply a DC voltage of a charging potential of Vth+VD to the charging roller.
This contact charging system, in which only DC voltage is applied to the contact charging member to thereby charge the object to be charged, will be referred to as the DC charging system.
This DC charging system involves, particularly in a low-humidity environment, image problems, such as xe2x80x9clateral stripes generated in halftone images, etc.xe2x80x9d mainly due to disturbance in the potential on the photosensitive drum (the photosensitive member) prior to charging, and what is called xe2x80x9ca drum positive ghostxe2x80x9d generated mainly due to a difference in the charging potential on the photosensitive drum in some cases.
As is known in the art, such image problems (lateral stripes generated in halftone images, etc., and a drum positive ghost) can be effectively prevented by providing a so-called charge removal means which irradiates the photosensitive drum with light before the charging process to thereby remove the residual charge.
As shown in FIG. 12, it has been general practice to provide the image forming apparatus main body with a charge removal means. For example, a charge removing device 302 (which consists of a chip array formed by arranging a plurality of LEDs 303, a fuse lamp or the like) is opposed to a photosensitive drum 1. FIG. 12 is a schematic perspective view of a charge removing device used in a conventional image forming apparatus.
However, this conventional construction, in which the charge removing device (a light source consisting of a chip array formed by arranging a plurality of LEDs, a fuse lamp or the like) is provided in the image forming apparatus main body so as to be opposed to the photosensitive drum, involves various problems. For example, the charge removing device is rather expensive, and the degree of freedom in the design of the image forming apparatus main body is restricted by the arrangement of the charge removing device.
In particular, in an image forming apparatus of the type in which a process cartridge is attached to and detached from the apparatus main body, consideration must be taken so that the location of the charge removing device may not interfere with the attachment/detachment of the process cartridge. Further, as a result of the attachment/detachment of the process cartridge, a problem is caused in that the positional accuracy of the charge removing device and the photosensitive drum deteriorates.
It might be possible to provide the charge removing device on the process cartridge side. However, that would inevitably make the process cartridge more expensive. Further, due to the provision of electrical contacts for connection with the image forming apparatus main body, the apparatus would become rather complicated and more expensive.
It is an object of the present invention to provide an image forming apparatus and a process cartridge capable of preventing image problems, such as lateral stripes generated in halftone images and drum positive ghost, without involving a complicated structure of the apparatus, an increase in cost, or restriction in design.
Another object of the present invention is to provide an image forming apparatus and a process cartridge having an image bearing member, and a light guide means for guiding light, wherein the light guide means has a reflection means for reflecting light to the image bearing member side.
Still another object of the present invention is to provide an image forming apparatus and a process cartridge having an image bearing member, and a light guide means for guiding light, wherein the light guide means changes the direction of light from the light source to the image bearing member.
Further objects of the present invention will become apparent from the following description.